This invention relates to laser electrophotographic printers and more specifically relates to an aspheric lens for providing improved mirror tilt error correction and scan bow correction in such printers.
Electrophotographic printers are designed to create an image by placing a series of picture elements (pels) on an image receiving material. For example, an image may be created by a light source, such as one or more lasers, which reflect from a multi-faceted or polygon mirror or galvo scanning mirror for scanning across a photoconductor surface in a succession of scan lines. The light beam or beams place a series of overlapping pels on the photoconductor surface. Each pel is placed in a pel area and the light beam is modulated so that some pel areas are exposed to light and some are not. Whenever a pel containing light strikes the photoconductor surface, the surface is discharged at that pel location. In this manner, the photoconductor surface is caused to bear a charge pattern of pels which matches the object image that is to be reproduced. The printed copy is obtained by developing the charge pattern and transferring the developed image to print material, usually paper.
Electrophotographic printers are well known in the art and are described, for instance, in U.S. Pat. No. 4,544,264 issued to Larry W. Bassetti, dated Oct. 1, 1985 and U.S. Pat. No. 4,625,222, issued to Larry W. Bassetti et al, dated Nov. 25, 1986, both assigned to the assignee of the present invention.
An inherent error associated with multi-faceted scanners is the tilt of the facet surface relative to the rotating axis. If the pitch varies from facet to facet a reflecting beam will be angularly displaced by twice the tilt angle. Uncorrected, the displacement or tilt error is typically 200 to 300 times that which is considered acceptable in electrophotographic printers.
In the prior art, conventional cylindrical optic systems have been used to provide partial correction for the tilt error. Such conventional optic systems have been limited in their ability to maintain a conjugate relationship between the facet surface and the photoconductor surface for the entire scan.
Prior systems contain tilt correcting lens. In such systems, as the laser beam scans across the photoconductor surface the distance from the polygon mirror in the photoconductor surface varies by the inverse cosine of the scan angle theta. These prior systems make use of either a wrapped toric lens or a long cylindrical lens disposed in proximity to the photoconductor surface for performing the tilt correction.
The use of constant optical power cylindrical lenses near the image plane for providing tilt correction is known. However, multispot laser printers, for example, are not amenable to constant optical power lens for providing such tilt connection. Using a constant power lens, the scan lines will bow, i.e., not be perfectly straight or flat. The bow in the scan line can be as much as 20 to 40 microns. Moreover, the magnitude and/or direction of bow in adjacent scans vary. In a typical multispot laser printer the desired spacing between lines is 50 microns. Therefore, the scan bow error effect is objectionable in high quality printing. A prior art solution to correct for tilt error by use of a constant power lens, is to bend the lens into an arc.
When using a toroidal surface lens, the lens is disposed so that the object distance, S, (from the polygon mirror to the lens) and the image distance, S', (from the lens to the photoconductor surfaces) is divided so that 1/S +1/S'=1/F, where F is the focal distance of the lens. Since the arcuate lens can only approximate the ideal surface needed, a compromise is obtained. The compromise suffers as the scan angle requirements are increased.
A long straight cylindrical lens makes use of the fact that when placed in proximity to the photoconductor surface, the above equation is more closely approximated. However, the long straight cylindrical lens approach suffers to an even greater extent than the wrapped cylindrical lens approach as the scan angle requirements are increased.
The present invention provides a simpler and less expensive solution to the problems of tilt error correction and bowing of the scan lines by use of an aspheric plastic molded lens. Once a mold is manufactured, lenses for providing tilt correction in multispot printers become less expensive and simple to manufacture.